NAME

perllexwarn - Perl Lexical Warnings

DESCRIPTION

The usewarnings
pragma enables to control precisely what warnings are
to be enabled in which parts of a Perl program. It's a more flexible
alternative for both the command line flag -w and the equivalent Perl
variable, $^W
.

This pragma works just like the strict
pragma.
This means that the scope of the warning pragma is limited to the
enclosing block. It also means that the pragma setting will not
leak across files (via use, require or do). This allows
authors to independently define the degree of warning checks that will
be applied to their module.

By default, optional warnings are disabled, so any legacy code that
doesn't attempt to control the warnings will work unchanged.

The code in the enclosing block has warnings enabled, but the inner
block has them disabled. In this case that means the assignment to the
scalar $c
will trip the "Scalar value @a[0] better written as $a[0]"
warning, but the assignment to the scalar $b
will not.

Default Warnings and Optional Warnings

Before the introduction of lexical warnings, Perl had two classes of
warnings: mandatory and optional.

As its name suggests, if your code tripped a mandatory warning, you
would get a warning whether you wanted it or not.
For example, the code below would always produce an "isn't numeric"
warning about the "2:".

With the introduction of lexical warnings, mandatory warnings now become
default warnings. The difference is that although the previously
mandatory warnings are still enabled by default, they can then be
subsequently enabled or disabled with the lexical warning pragma. For
example, in the code below, an "isn't numeric"
warning will only
be reported for the $a
variable.

Note that neither the -w flag or the $^W
can be used to
disable/enable default warnings. They are still mandatory in this case.

What's wrong with -w and $^W

Although very useful, the big problem with using -w on the command
line to enable warnings is that it is all or nothing. Take the typical
scenario when you are writing a Perl program. Parts of the code you
will write yourself, but it's very likely that you will make use of
pre-written Perl modules. If you use the -w flag in this case, you
end up enabling warnings in pieces of code that you haven't written.

Similarly, using $^W
to either disable or enable blocks of code is
fundamentally flawed. For a start, say you want to disable warnings in
a block of code. You might expect this to be enough to do the trick:

The other big problem with $^W
is the way you can inadvertently
change the warning setting in unexpected places in your code. For example,
when the code below is run (without the -w flag), the second call
to doit
will trip a "Use of uninitialized value"
warning, whereas
the first will not.

Lexical warnings get around these limitations by allowing finer control
over where warnings can or can't be tripped.

Controlling Warnings from the Command Line

There are three Command Line flags that can be used to control when
warnings are (or aren't) produced:

-w

This is the existing flag. If the lexical warnings pragma is not
used in any of you code, or any of the modules that you use, this flag
will enable warnings everywhere. See Backward Compatibility for
details of how this flag interacts with lexical warnings.

-W

If the -W flag is used on the command line, it will enable all warnings
throughout the program regardless of whether warnings were disabled
locally using nowarnings
or $^W =0
. This includes all files that get
included via use, require or do.
Think of it as the Perl equivalent of the "lint" command.

-X

Does the exact opposite to the -W flag, i.e. it disables all warnings.

Backward Compatibility

If you are used with working with a version of Perl prior to the
introduction of lexically scoped warnings, or have code that uses both
lexical warnings and $^W
, this section will describe how they interact.

How Lexical Warnings interact with -w/$^W
:

1.

If none of the three command line flags (-w, -W or -X) that
control warnings is used and neither $^W
or the warnings
pragma
are used, then default warnings will be enabled and optional warnings
disabled.
This means that legacy code that doesn't attempt to control the warnings
will work unchanged.

2.

The -w flag just sets the global $^W
variable as in 5.005. This
means that any legacy code that currently relies on manipulating $^W
to control warning behavior will still work as is.

3.

Apart from now being a boolean, the $^W
variable operates in exactly
the same horrible uncontrolled global way, except that it cannot
disable/enable default warnings.

4.

If a piece of code is under the control of the warnings
pragma,
both the $^W
variable and the -w flag will be ignored for the
scope of the lexical warning.

5.

The only way to override a lexical warnings setting is with the -W
or -X command line flags.

The combined effect of 3 & 4 is that it will allow code which uses
the warnings
pragma to control the warning behavior of $^W-type
code (using a local$^W=0
) if it really wants to, but not vice-versa.

Category Hierarchy

A hierarchy of "categories" have been defined to allow groups of warnings
to be enabled/disabled in isolation.

To determine which category a specific warning has been assigned to see
perldiag.

Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a
sub-category of the "syntax" category. It is now a top-level category
in its own right.

Fatal Warnings

The presence of the word "FATAL" in the category list will escalate any
warnings detected from the categories specified in the lexical scope
into fatal errors. In the code below, the use of time, length
and join can all produce a "Useless use of xxx in void context"
warning.

If you want to downgrade a warning that has been escalated into a fatal
error back to a normal warning, you can use the "NONFATAL" keyword. For
example, the code below will promote all warnings into fatal errors,
except for those in the "syntax" category.

Reporting Warnings from a Module

The warnings
pragma provides a number of functions that are useful for
module authors. These are used when you want to report a module-specific
warning to a calling module has enabled warnings via the warnings
pragma.

The call to warnings::register
will create a new warnings category
called "MyMod::Abc", i.e. the new category name matches the current
package name. The open function in the module will display a warning
message if it gets given a relative path as a parameter. This warnings
will only be displayed if the code that uses MyMod::Abc
has actually
enabled them with the warnings
pragma like below.

It is also possible to test whether the pre-defined warnings categories are
set in the calling module with the warnings::enabled
function. Consider
this snippet of code:

package MyMod::Abc;

sub open {

warnings::warnif("deprecated",

"open is deprecated, use new instead");

new(@_);

}

sub new

...

1;

The function open has been deprecated, so code has been included to
display a warning message whenever the calling module has (at least) the
"deprecated" warnings category enabled. Something like this, say.

Either the warnings::warn
or warnings::warnif
function should be
used to actually display the warnings message. This is because they can
make use of the feature that allows warnings to be escalated into fatal
errors. So in this case

the warnings::warnif
function will detect this and die after
displaying the warning message.

The three warnings functions, warnings::warn
, warnings::warnif
and warnings::enabled
can optionally take an object reference in place
of a category name. In this case the functions will use the class name
of the object as the warnings category.